Optical communications systems using optical fibers to couple a light source. Such as laser, and a photodetector are now widely used for high speed (for example, Gbit/sec data rates) and long distance (for example, trans-Atlantic or trans-Pacific) communications. Many technical problems have had to be overcome for these systems to reach their present state of development. Perhaps the most widely known problem was caused be the lossy nature of the first silica based optical fibers. The loss in such fibers was greatly reduced, to the order of a tenth of a dB/km or even less, by the development of fibers fabrication techniques that greatly reduced the presence of loss-creating impurities in the fibers.
After low loss optical fibers had been developed, other system parameters became important to the further development of optical communications systems. For example, fibers have chromatic dispersion; that is, the propagation velocity of the radiation depends upon its frequency. Narrow band light sources in the form of solid state lasers were developed. These lasers typically radiated in several relatively closely spaced modes which propagated at different velocities. The presence of multiple modes and the existence of chromatic dispersion limited either the data transmission rate of the transmission distance. Radiation sources, such as distributed feedback (DFB) lasers, that emitted only a single mode were developed to overcome these problems.
However, even the single mode modulated light of a DFB-laser has a finite bandwidth which causes a pulse to spread when chromatic dispersion is present. One approach to solving this problem was served by the development of dispersion shifted fibers, which are often referred to by the acronym DSF. Dispersion shifted fibers have a region of very low or no chromatic dispersion. However, the use of such fibers suffers from several drawbacks. Firstly, the laser must be selected to emit at the frequency at which the fiber has no chromatic dispersion. Secondly, much non-dispersion shifted fiber has already been installed.
Other techniques that compensate for fiber chromatic dispersion are desirable if they overcome the previously discussed limitations imposed be non-dispersion shifted fibers. One technique inserts, at an arbitrary point in the transmission path between the transmitter and the receiver, a dispersion compensating fiber (DCF). The length of fiber is selected to provide dispersion compensation for a certain transmission length and therefore enable transmission over either an extended distance or at a higher rate. This approach suffers from the added cost of the DCF and, more significantly, the losses introduced by such fibers. The losses are at least comparable to the losses in the system fibers and limit the system capabilities.
An apparatus which reduces the costs of the DCF and compensates for the losses introduced by the DCF is known from U.S. Pat. No. 5,404,413, issued Apr. 4, 1995 to Jean-Marc Delavaux et al. Under the title "Optical Circulator for Dispersion Compensation," commonly assigned herewith. (A corresponding European Patent Application EP 0 658 988 A1 was published on Jun. 21, 1995.). The apparatus has an optical circulator with at least first, second and third ports. The apparatus also has return means and a dispersion compensating waveguide, such as a DC-fiber, connecting the return means to the second port. An amplifier is connected to the circulator. The amplifier has a pump laser, a multiplexer, and a doped fiber. The pump laser is connected to the multiplexer, and the fiber amplifier is connected between the return means and the second port of the circulator.